WS6: Finalize the public API before GA

## Summary

The public API must be settled before `1.0.0`, because only backwards compatible growth is permitted after GA. The Azure SDK architecture board review is not complete, no archived API surface artifact exists, the error model carries a raw AMQP condition string with no reason enum, several transport options that .NET exposes are absent, and `vcpkg.json` declares a wrong version and an unused dependency.

Severity tags follow the key in #7252.

## Motivation

The Azure SDK release checklist requires an architecture board API review and an archived API surface artifact. .NET keeps that artifact as `sdk/eventhub/Azure.Messaging.EventHubs/api/Azure.Messaging.EventHubs.netstandard2.0.cs`. The C++ package has no equivalent.

`EventHubsException` carries a raw condition string, a description, an `IsTransient` flag, and an optional status code (`inc/azure/messaging/eventhubs/eventhubs_exception.hpp:38-65`). .NET maps errors onto a `FailureReason` enum with eleven values (`sdk/eventhub/Azure.Messaging.EventHubs/src/EventHubsException.cs:221-255`), which gives a caller something to act on without parsing a string.

The C++ versioning guidelines state "DO be 100% backwards compatible with older versions of the same package" [`cpp-versioning-backwards-compatibility`] and "DO introduce a new package (with new assembly names, new namespace names, and new type names) if you must do an API breaking change" [`cpp-versioning-new-package`]. The C++ guidelines record no ABI or binary compatibility promise, so the constraint is source compatibility.

A sweep of the C++ options structs did not find a per-attempt timeout, a retry mode, WebSocket transport, a proxy, or a custom endpoint, all of which .NET exposes through `EventHubsRetryOptions` and `EventHubConnectionOptions`. That sweep was not exhaustive, so each item needs a decision rather than an assumption.

`vcpkg.json` declares `version-string: 1.0.0` while the package is at a beta version, and it declares an `opentelemetry-cpp` dependency with the `otlp-http` feature that the library sources do not use (`vcpkg.json:3-11`). Note that #7259 may make that dependency real.

## Proposal

- [ ] Complete the Azure SDK architecture board API review for `azure-messaging-eventhubs` and `azure-messaging-eventhubs-checkpointstore-blob`. Produce and archive the API surface listing, the C++ analog of the .NET `api/*.cs` files. **[GA blocker]**
- [ ] Decide whether the reason enum supplements the condition string or replaces it. A replacement is a breaking change and must land before `1.0.0`. **[GA blocker to decide]**
- [ ] Add a reason enum mapped from AMQP conditions, modeled on `EventHubsException.cs:221-255`, as a new member alongside the existing fields (`inc/azure/messaging/eventhubs/eventhubs_exception.hpp:38-65`). **[GA quality bar]**
- [ ] Sweep `ConsumerClientOptions` and `ProducerClientOptions` against the .NET `EventHubsRetryOptions` and `EventHubConnectionOptions`. Decide each missing option as add-before-GA, add-after-GA, or never. **[GA quality bar]**
- [ ] Correct `vcpkg.json`: fix `version-string`, and remove or justify the unused `opentelemetry-cpp` dependency (`vcpkg.json:3-11`). **[GA quality bar]**

## Resolution of the earlier open question

This issue previously tagged the reason enum **GA quality bar** on the unverified assumption that the C++ SDK permits additive API growth after `1.0.0`. The guidelines were then read directly, and they permit it: `cpp-versioning-backwards-compatibility` constrains source compatibility, and adding a new enum type plus a new accessor to `EventHubsException` keeps every existing caller compiling.

The tag therefore stays **GA quality bar**, but it holds only for a supplement. Removing or repurposing the existing condition string breaks callers and must land before `1.0.0`. That is why the supplement-or-replace decision is split out above as its own blocker.

One caveat: the guidelines record no ABI promise, so adding a member to the exception class is safe for source builds but changes the class layout. Consumers who link a prebuilt binary must rebuild. Record that expectation with the decision.

## Validation

- [ ] The architecture board sign-off is recorded and the API surface artifact is archived in the repository.
- [ ] The supplement-or-replace decision is recorded in this issue.
- [ ] Every option in the sweep carries a recorded decision.
- [ ] `vcpkg.json` declares the correct version and no unused dependency.